This invention relates to a wire-breakage detecting apparatus mounted in e.g. a motor vehicle, for detecting failure such as wire breakage in a circuit which is driven in order to detect a thermal resistance and thermal conductivity of liquid by a bridge circuit in determining whether or not a urea water for a urea SCR (Selective Catalyst Removing method) catalyst for decomposing NOx contained in an exhaust gas into water and nitrogen has an appropriate urea density.
Traditionally, as a urea density sensor for an exhaust gas cleaning system for e.g. a diesel vehicle, by amplifying a minute signal of 0.1 mV as a sensor signal, A/D converting it by a microcomputer and supplying a signal indicative of a urea density value to the exhaust gas cleaning system, the exhaust gas cleaning system is monitored if it operates appropriately. As a technology used for such a urea density sensor, a power source circuit is employed and the voltage is supplied from the power source circuit to a heater resistor.
Further, traditionally, as a failure detecting technique for failure detection between the above power source circuit and heater resistor, as described in Patent Reference 1 identified below, a technique such as a comparator abnormality detecting device, which can detect the abnormality due to failure of a peripheral device as well as the comparator, is known.
Patent Reference 1: JP-A-2003-248022
Meanwhile, in view of the trend of exhaust gas control in recent years, it is necessary to severely control the specific component contained in the exhaust gas. Therefore, it has been eagerly demanded to measure periodically the exact urea density by application of a voltage to the heater resistor. Thus, it has been demanded to provide an exact output voltage from the power source circuit to the sensor resistor and accurately detect failure of the circuit portion connected from the power source circuit to the sensor resistor.